Inkjet printers eject patterns of ink drops to form both single and multicolor printed images. In an inkjet printer, one or more printheads eject drops of ink onto an image receiving surface, such as paper or an indirect image receiving member, and the patterns of individual ink drops give the appearance of text, graphics, and other images. Combinations of multiple ink colors, such as cyan, magenta, yellow, and black (CMYK) inks, can form a wide range of perceptible colors in a printed image.
Many modern inkjet printers receive digital data corresponding to a printed image. The digital data for a printed image often include data corresponding to printed colors that are encoded in a continuous tone (contone) format. In a contone format, a single two-dimensional location in the image, which is referred to as a contone pixel, can have a wide range of colors that are formed from different intensity levels of basic colors such as red, green, blue (RGB) colors. In other embodiments, the colors of contone pixels are encoded in a device independent color space such as the L*a*b* color space or other color spaces known to the art. Video display devices often reproduce colors using contone pixels. Inkjet printers, however, produce images from patterns of ink drops using a comparatively small number of inks in a halftone output that includes a pattern of printed ink drops. For example, the color of a single pixel of contone image data may be reproduced using a pattern of multiple separate ink drops that reproduce the contone color on a physical print medium. The physical properties of the ink drops affect the perceptible intensity of printed ink colors since the ink drops spread on the image receiving surface and generally have circular shapes instead of the square shapes associated with contone pixels.
Inkjet printers employ tone reproduction curves (TRCs) to convert image data that are provided in the contone formats to values such that when the values are halftoned, the printed ink patterns provide an accurate reproductions of the original contone images. For example, a set of gray contone pixels with a relative level of 50% between pure-black (100% level) and pure-white (0% level) generates a halftone pattern with half of the pixels being assigned a black ink drop and the other half of the pixels being left blank, which corresponds to white when printing on white paper. Because the ink drops spread on the image receiving surface, however, printing the direct halftone pattern produces a printed image that is darker than the intended 50% level in one printer configuration. A digital controller in the printer uses the TRC to generate modified contone image data that includes adjustments for the intensity levels of some or all of the contone pixels in the original digital image data. For example, in one configuration a TRC reduces a contone pixel levels from 50% to 40%. The printer then uses halftone processes that are known to the art to convert the modified contone image data into patterns of image data corresponding to printed ink drops that form the printed image.
Some inkjet printers are capable of printing with ink drops of different sizes or using a range of inks that have different colorant concentrations within the same image. The TRC's for images that are printed using a combination of different ink drop sizes or different colorant concentrations are hard to calibrate and can show uneven or inaccurate halftone printed images. Existing techniques to correct the image uniformity that ignore the multiple drop sizes may not be robust to noises produced by these multiple drop sizes and may not give optimal uniformity. Consequently, improved systems and methods for controlling halftone printing using multiple ink drop sizes or colorant concentration levels to enable accurate color reproduction would be beneficial.